Planar waveguide and a process for its fabrication

ABSTRACT

A planar waveguide and a process for making a planar waveguide is disclosed. The waveguide has a layer of dope host material formed on a substrate. The host material is a trivalent material such as a metal fluoride, wherein the metal is selected from the Group III B metals and the lanthanide series rare earth metals of the Mendeleevian Periodic Table. The dopant is a rare earth metal such as erbium. The waveguide has an emission spectrum with a bandwidth of about 60 nm for amplification of an optical signal at a wavelength of about 1.51 μm to about 1.57 μm. The waveguide is made by forming the layer of doped host material on a substrate. The film is formed by evaporating materials from two separate sources, one source for the dopant material and a separate source for the host material and forming a film of the evaporated materials on a substrate.

This is a division of application Ser. No. 08/373,346 filed Jan. 17,1995, now U.S. Pat. No. 5,555,342.

FIELD OF THE INVENTION

This invention relates to optical amplifiers and, in particular, planaroptical amplifiers involving rare earth doped materials.

ART BACKGROUND

Considerable recent research has involved the development of opticalamplifiers useful in optical communications. Typically, these amplifiersinvolve a waveguide formed in a glassy material (a material that has nolong-range ordering and is characterized by an absence of Bragg peaks inX-ray diffraction and/or a glass transition observed in differentialscanning calorimetry) with a rare earth dopant present in the waveguidecore and with a region of lower refractive index surrounding the core.Generally, the glassy host material does not substantially affect theemission spectrum of the dopant, and the rare earth dopant material ischosen to have a spectral emission line corresponding to a wavelength atwhich optical communication is to be performed. For example, mostlong-haul optical communication is performed either at 1.3 μm or 1.55μm. Optical devices that amplify signals at 1.3 μm are described in U.S.Pat. No. 5,140,658 to Sunshine et al.

Optical amplifiers at 1.55 μm have been demonstrated and are describedin U.S. Pat. No. 5,119,460 to Bruce et al. These amplifiers involve awaveguide fiber having erbium, that emits at 1.52 to 1.56 μm, present inthe core at concentrations typically in the range 10 to 1000 parts permillion. During operation of the amplifier, optical power at awavelength 0.975 or 1.48 μm is introduced into the waveguide core alongwith a signal at the 1.55 μm wavelength. The optical power induces atransition in the erbium that populates a state, the ⁴ I_(13/2) state,capable of stimulated emission around 1.55 μm, and the signal inducesthis transition from the populated state. Thus, the output from theamplifier involves a signal at 1.55 μm that has an intensity approachingthat of the combined power and signal inputs. In his manner, an opticalsignal is amplified, in contrast to electrical amplification involvingconversion of the optical signal to an electrical signal, followed byelectrical amplification and another conversion back to an opticalsignal.

The concentration of the dopant affects the efficiency of the amplifier.Since the properties of the amplifier depend upon the absolute number ofdopant atoms in the host material, the dopant concentration that isnecessary for adequate performance depends upon the length of thedevice. For example, the dopant concentration in fiber amplifiers ismuch less than the dopant concentration in planar optical amplifiers,because fiber amplifiers are much longer than planar optical amplifiers.However, high dopant concentrations lead to concentration quenching ofthe luminescence from the dopant. If such quenching occurs, theamplifier gain is reduced and the amplifier performance is consequentlydegraded. Therefore, planar optical amplifiers that amplify signals at1.55 μm and that overcome the problems associated with high dopantconcentration, and a process for making such planar optical waveguides,are sought.

SUMMARY OF THE INVENTION

The present invention contemplates a planar optical waveguide thatamplifies an optical signal at a wavelength from about 1.51 μm to about1.57 μm. The planar optical waveguide contains a region suited forguiding the signal comprising a doped, waveguide host material. Thewaveguide host material is either a polycrystalline or a singlecrystalline material with trivalent cations. It is advantageous if thehost material is a trivalent material such as a fluoride of a IIIB metalor a rare earth metal (lanthanide series) from the Mendeleevian PeriodicTable. For example, the material is lanthanum fluoride (LaF₃), yttriumfluoride (YF₃), or lutetium fluoride (LuF₃). The waveguide host materialis doped with other rare earth ions. It is advantageous if the dopant iserbium (Er).

It is advantageous if the dopant concentration in the host material isabout 0.05 atomic percent to about 12 atomic percent in planarwaveguides that are about 1 cm to about 20 cm in length. Since theconcentration of dopant may vary through the thickness of the hostmaterial, it is advantageous if the maximum dopant concentration iswithin this range. In a preferred embodiment, the maximum dopantconcentration is about 4 to about 5 atomic percent.

The dopant concentration through the thickness of the host material iseither constant or varied. If varied, it is advantageous if the dopantprofile matches the intensity profile of the light transmitted throughthe waveguide. In this regard, it is also advantageous if the maximumdopant concentration is at or near the center of the waveguide.

The amplifier of the present invention significantly amplifies a signalover a broad band. Significant amplification means that the intensity ofthe signal throughout the entire bandwidth is not less than one-third ofthe peak intensity. For example, the planar waveguide doped with erbiumas previously described has an emission spectrum spanning a range from1.51 μm to 1.57 μm in wavelength, giving a 60 nm bandwidth. Furthermore,the amplifier of the present invention provides an environment in whichthe lifetime of the spontaneous luminescent emission from the hostmaterial, as measured according to the description given below, is atleast about 1 ms. Since longer lifetimes provide a better environmentfor signal amplification, it is advantageous if this lifetime is atleast about 10 ms.

To fabricate the planar optical waveguide, a layer of waveguiding hostmaterial is formed on a substrate. The substrate has a refractive indexthat is lower than the refractive index of the waveguiding hostmaterial. If the substrate does not have a refractive index that islower than the refractive index of the waveguiding host material, abuffer layer of a material with a suitable refractive index is formedbetween the substrate and the waveguiding host material. The planarwaveguide is adapted to receive an optical signal and to receive powerto amplify the optical signal. The waveguide is further adapted tooutput a signal that is an amplified input signal.

Examples of suitable substrates on which the host material is formedinclude single crystalline quartz substrates, fused quartz substrates,aluminum oxide substrates, calcium fluoride substrates or siliconsubstrates. Since it is advantageous if the substrate's coefficient ofthermal expansion matches that of the waveguiding host material, singlecrystalline quartz substrates and aluminum oxide substrates areadvantageous in this regard. If the substrate has a higher refractiveindex than the waveguide, a film that forms an optical buffer layer isformed on the substrate before the film of the host material is formedthereon. For example, if the substrate is a silicon substrate, thebuffer layer is a material that has a refractive index lower than therefractive index of the waveguide. Silicon dioxide is an example of asuitable buffer layer material. The silicon dioxide layer is formed onthe silicon substrate using conventional techniques.

The layer of doped host material is then formed on the substrate. It isadvantageous if the trivalent host material and the dopant are depositedfrom separate sources, using conventional apparatus such as evaporationovens, electron beam evaporators, and the like. In one embodiment, thetrivalent host material is formed using LaF₃ , YF₃ or LuF₃ as sourcematerial and the dopant is introduced using ErF₃ as source material.Sources and techniques for depositing films of the host and dopantmaterials specified above onto substrates are well known to thoseskilled in the art. It is advantageous if the temperature of thesubstrate during the formation of the doped, host material layer isabout 300° C. to about 600° C.

The thickness of the host material layer so formed is a matter of designchoice. Film thicknesses of about 0.8 μm to about 2 μm are contemplatedas suitable. Film thicknesses greater than 2 μm are also contemplated toreduce coupling losses to fibers with larger cores.

Because independent sources are used for the layer of host material andthe dopant material, the concentration of the dopant in the hostmaterial is widely variable. For example, it is contemplated that themaximum concentration of ErF₃ in LaF₃ host material is from about 0.05atomic percent to about 20 atomic percent in the optical devices of thepresent invention. The concentration of the dopant is variablethroughout the thickness of the layer of the host material in aparticular device. As previously mentioned, if the dopant concentrationvaries through the thickness of the host material, then the maximumdopant concentration in the layer is found near the center of thewaveguiding layer. Such a dopant profile is advantageous because themaximum mode intensity of the light occurs also at the center of thewaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate planar waveguides of the present invention.

FIGS. 3 and 4 illustrate ridge waveguides of the present invention.

FIG. 5 illustrates a photoluminescence spectrum of a waveguide of thepresent invention.

FIG. 6 illustrates an emission lifetime of a waveguide of the presentinvention.

DETAILED DESCRIPTION

As discussed, the invention is directed to a planar waveguide whichamplifies an optical signal at a wavelength of about 1.51 μm to about1.57 μm. The present invention also contemplates a process for makingsuch a waveguide.

The planar optical waveguide contains a region suited for guiding thesignal comprising a layer of a doped host material. Er is an example ofa suitable dopant.

The host material is chosen to match the valence state of the dopant,such that the dopant substitutes for a host cation rather than occupyinginterstitial sites. The symmetry of the site at which dopant atoms areintroduced as well as the size and oxidation state of thehost-constituent-atom being replaced by the dopant determines theallowed oxidation states of the dopant atoms. Site symmetries, exemplaryhost materials having those symmetries, and the other properties thatlead to a desired oxidation state are well known and are tabulated incompendia such as Laser Crystals, Alexander Kaminskii, Springer-Verlag,1981, and the Major Ternary Structural Families, Miller & Roy,Springer-Verlag, 1974.

Thus, for example, Er³ + are maintained in this valence state byintroduction into a film of a trivalent host material that is, forexample, a fluoride of a IIIB metal or a rare earth metal from theMendeleevian Periodic Table. Examples of these host materials includeYF₃, LaF₃, and LuF₃. The structure of the host material is either singlecrystalline, polycrystalline or amorphous. Polycrystalline materialshave certain processing advantages because they are more easilyproduced.

The gain of the amplifier is proportional to the concentration of thedopant in the host. Dopants are introduced in the concentration range ofabout 0.05 to about 12 atomic percent. Concentrations of less than about0.05 atomic percent typically lead to undesirably low gain in theamplifier. Although concentrations above about 2 atomic percent aretypically avoided because the possibility of concentration quenching isenhanced at these concentrations, the present invention contemplatesdopant concentrations of up to about 12 atomic percent in thewaveguiding host material. The problems associated with concentrationquenching are reduced because the dopant is introduced substitutionallyinto the host material. Consequently, fewer charge compensating defectsarise in these materials than in materials in which the dopant occupiesinterstitial sites in the host material. This in turn reduces the ionclustering, which would otherwise occur at such high dopantconcentrations and which would deteriorate amplifier performance. In apreferred embodiment the dopant concentration is about 4 atomic percentto about 5 atomic percent.

The waveguides of the present invention are illustrated in FIGS. 1-4.FIG. 1 illustrates a planar waveguide 10 formed on a substrate 20. Asindicated by the shaded area 30 the layer 10 is doped through its entirethickness. The layer 10 is formed directly on a suitable substrate 20.Quartz, fused quartz, aluminum oxide, silicon and calcium fluoride areexamples of suitable substrate materials.

In the alternate embodiment pictured in FIG. 2, the waveguide has abuffer layer 40 formed between the substrate layer 80 and the waveguidematerial 70. The buffer layer 40 provides optical separation between thesubstrate 80 and the waveguide layer 70 when the refractive index of thesubstrate 80 is higher than that of the waveguide material 70. In suchan instance, the buffer layer 40 is made of a material with an index ofrefraction that is lower than the index of refraction of the waveguidematerial 70. One skilled in the art will recognize that, to achieveoptical separation, the index of refraction of the buffer layer need beonly nominally lower than the index of refraction of the waveguidematerial. For example, if the substrate 80 is made of a material with ahigh index of refraction such as silicon, it is advantageous if thewaveguide has a buffer layer. Silicon dioxide, which has an index ofrefraction that is lower than that of the trivalent host materials ofthe present invention, is an example of a suitable buffer layermaterial. FIG. 2 also depicts a waveguide in which the doping profile ofthe waveguiding material 70, indicated by the shaded area 60, variesthrough the thickness of the waveguiding layer.

The waveguide layer (10 in FIG. 1) is formed in one embodiment byplacing the substrate in a chamber. The chamber is evacuated. Separatesources for the host and dopant materials are provided so that theamount of material from each source that is incorporated into thewaveguide layer 10 is controlled. For example, the host and dopantmaterial are each placed in a separate Knudsen oven that is commerciallyobtained from EPI of Saint Paul, Minn. The composition of the layer iscontrolled by controlling the amount of material that is evaporated fromeach source. Increasing or decreasing the temperature of the oven inwhich the dopant source is placed will correspondingly increase ordecrease the dopant concentration in the waveguide layer. The dopantconcentration is varied through the thickness of the waveguide layer bymodulating the temperature of this oven as the waveguiding layer isformed. A similar effect is obtained by individually controlling theoven shutters which controls the amount of material flowing from theoven and into the chamber.

In one embodiment wherein a waveguide layer with a composition of 5atomic percent ErF₃ and 95 atomic percent LaF₃ is desired, thetemperatures of the Knudsen ovens are adjusted so that for 5 units ofErF₃ evaporated, 95 units of LaF₃ are evaporated. To form a waveguidelayer with this composition, the oven temperature for LaF₃ is set atabout 1360° C. and the oven temperature for ErF₃ is set at about 1150°C.

In an alternate embodiment, after the waveguide layer with the desiredcomposition is formed on the substrate, the layer is patterned to form aridge waveguide. Such a waveguide is illustrated in FIG. 3. To patternthe waveguide layer, a material such as a photoresist (not shown) isfirst formed on the waveguide layer. The photoresist is patterned byconventional techniques such as lithography in conjunction with etchingto form the configuration shown in FIGS. 3 and 4. (Lithographic andetching techniques are described respectively in Nishihara et al.,Optical Integrated Circuits, McGraw-Hill 1985). The ridge waveguideillustrated in FIG. 3 results when the waveguide layer is formeddirectly over the substrate 200 and patterned as described above to forma ridge 220. The ridge waveguide illustrated in FIG. 4 results when theridge 330 is formed over a buffer layer 310, which is formed over asubstrate 300.

In one embodiment of the present invention, the dopant concentration inthe waveguide layer is uniform through the thickness of the waveguidelayer. In an alternate embodiment, the dopant profile is matched to theintensity profile of the light that travels through the waveguide. Inthe latter embodiment the dopant profile through the waveguide is suchthat the maximum dopant concentration in the host material is found at apoint about equidistant from the top and the bottom of the planarwaveguide. This dopant profile is depicted as 60 in FIG. 2 and 210 inFIG. 3.

The resulting amplifiers are useful for amplification of signalsassociated with optical communications. Nevertheless, other applicationssuch as high power optical amplifiers contemplated for use in cabletelevision systems are possible and are not precluded. Insertion ofsignal and amplification power is accomplished by expedients such asdescribed in Integrated Optics Theory and Technology by Hunsperger,Springer-Verlag, 1982. Output signals are coupled to the waveguideamplifier by methods such as those described in Hunsperger. Typicalapproaches for input and output coupling include the use of an input andoutput silica optical fiber butted to the waveguide region of theamplifier.

The following examples are illustrations of specific embodiments of theclaimed invention.

EXAMPLE 1

A silicon wafer with a 10 μm thick layer of silicon dioxide formedthereon was placed in a vacuum chamber. The chamber was evacuated to apressure of about 10⁻⁹ Torr. Sources of LaF₃ and ErF₃ were each placedin a separate Knudsen ovens (EPI, St. Paul, Minn.). The ovens wereheated to a temperature of about 1360° C. for LaF₃ and 1150° C. forErF₃. The pressure during the deposition was maintained at or below 10⁻⁷Torr. During deposition the temperature of the ovens was controlled by aclosed loop feedback system, using a tungsten/rhenium thermocouple. Fora uniform composition throughout the film, the temperatures of theKnudsen cell ovens were kept constant during the formation of the layer.The resulting film had a composition that was about five mole percentErF₃ and 95 mole percent LaF₃ uniformly throughout the layer.

The substrate temperature was held at about 550° C. The resulting filmthickness was about 0.8 μm. The film thickness was measured using aDEKTAK™ instrument. The composition and film thickness wereindependently determined using Rutherford Backscattering Spectrometry.The resulting films were polycrystalline.

The resulting waveguides transmitted signals with wavelengths of about0.20 μm to about 20 μm. A photoluminescence spectrum of a planarwaveguide was measured using the 514.5 nm line of an argon laser, with apower of about 400 mW and a beam diameter of about 0.5 mm. Theluminescence was spectrally analyzed with a single-gratingmonochromator, and the signal was detected with a liquid nitrogen cooledgermanium detector. The pump beam was chopped at a frequency of 11 Hz,and the signal was amplified using a lock-in amplifier. Thephotoluminescence is depicted in FIG. 5 as a function of wavelength. Asdemonstrated by FIG. 5, the planar amplifier significantly amplifiedsignals in the band of about 1.51 μm to about 1.57 μm. Thus theamplifier demonstrated a broad (about 60 nm) bandwidth of significantamplification.

The lifetime of the luminescence as a function of the concentration ofthe dopant in the film was measured by monitoring the decay of theluminescence of the waveguide layer made according to the above exampleafter switching off the light source used to photo-excite the films.These lifetimes as a function of Er concentration in the film areillustrated in FIG. 6. The measured lifetime of the luminescence in afilm that contained 5 atomic percent Er decayed exponentially with atime constant of 12.8 ms. This relatively long lifetime illustrates thatthe waveguide layer provides a good environment for amplification.

We claim:
 1. A process for fabricating a planar waveguidecomprising:forming a layer of a doped, waveguiding host material and adopant material on a substrate by evaporating the host material from afirst source and the dopant material from the second source, wherein thefirst source material is a metal fluoride wherein the metal is selectedfrom the group consisting of the Group IIIB metals and the lanthanideseries rare earth metal of the Mendeleevian Periodic Table of theElements and the second source material is ErF₃.
 2. The process of claim1 wherein the concentration of the dopant in the host material is about0.05 atomic percent to about 5 atomic percent.
 3. The process of claim 1wherein the dopant is erbium and the doped, waveguiding host materialhas an emission spectrum with a bandwidth of about 60 nm at a signalwavelength of about 1.51 μm to about 1.57 μm.
 4. The process of claim 1further comprising controlling the dopant concentration in the doped,waveguiding material such that the dopant profile conforms to theintensity profile of the light transmitted through the doped,waveguiding material.
 5. The process of claim 3 wherein the first sourcematerial is selected from the group consisting of LaF₃,YF₃, and LuF₃. 6.The process of claim 5 wherein the doped, waveguiding material is formedon the substrate by heating a source of LaF₃ to a first temperature andheating a source of ErF₃ to a second temperature wherein the first andsecond temperatures are selected to control the rates of evaporation ofLaF₃ and ErF₃ from their respective sources such that for every 5 unitsof ErF₃ that are evaporated, 95 units of LaF₃ are evaporated therebyforming a doped waveguiding material that is about 95 atomic percentLaF₃ and about 5 percent ErF₃ on the substrate.
 7. The process of claim6 wherein the substrate is selected from the group consisting ofsilicon, crystalline quartz, fused quartz, calcium fluoride, andaluminum oxide.
 8. The process of claim 7 wherein the silicon substratehas a buffer layer of silicon dioxide formed thereover before the layerof doped, waveguiding material is formed on the substrate.